Large capacity acid or base generator and method of use

ABSTRACT

Method and apparatus for generating an acid or base, e.g. for chromatographic analysis of anions. For generating a base the method includes the steps of providing a cation source in a cation source reservoir, flowing an aqueous liquid stream through a base generation chamber separated from the cation source reservoir by a barrier (e.g. a charged membrane) substantially preventing liquid flow while providing a cation transport bridge, applying an electric potential between an anode cation source reservoir and a cathode in the base generation chamber to electrolytically generate hydroxide ions therein and to cause cations in the cation source reservoir to electromigrate and to be transported across the barrier toward the cathode to combine with the transported cations to form cation hydroxide, and removing the cation hydroxide in an aqueous liquid stream as an effluent from the first base generation chamber. Suitable cation sources include a salt solution, a cation hydroxide solution or cation exchange resin.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 11/502,294 filed on Aug.9, 2006, which is a continuation of application Ser. No. 10/177,080,filed on Jun. 21, 2002 (now abandoned), which is a divisional ofapplication Ser. No. 09/612,074, filed on Jul. 7, 2000 (U.S. Pat. No.6,682,701, issued on Jan. 27, 2004), which is a divisional ofapplication Ser. No. 09/017,050, filed on Feb. 2, 1998 (U.S. Pat. No.6,225,129, issued on May 1, 2001).

BACKGROUND OF THE INVENTION

The present invention relates to a large capacity apparatus forgenerating a high purity acid or base particularly for use as achromatography eluent, and to a method of using the apparatus.

In liquid chromatography, a sample containing a number of components tobe separated is directed through a chromatography separator, typicallyan ion exchange resin bed. The components are separated on elution fromthe bed in a solution of eluent. One effective form of liquidchromatography is referred to as ion chromatography. In this knowntechnique, ions to be detected in a sample solution are directed throughthe separator using an eluent containing an acid or base and thereafterto a suppressor, followed by detection, typically by an electricalconductivity detector. In the suppressor, the electrical conductivity ofthe electrolyte is suppressed but not that of the separated ions so thelatter may be detected by the conductivity detector. This technique isdescribed in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019and 3,956,559.

There is a general need for a convenient source of high purity acid orbase for use as an eluent for liquid chromatography and, particularly,for ion chromatography. In one technique, described in U.S. Pat. No.5,045,204, an impure acid or base is purified in an eluent generatorwhile flowing through a source channel along a permselective ionexchange membrane which separates the source channel from a productchannel. The membrane allows selective passage of cations or anions. Anelectrical potential is applied between the source channel and theproduct channel so that the anions or cations of the acid or base passfrom the former to the latter to generate therein a base or acid withelectrolytically generated hydroxide ions or hydronium ions,respectively. This system requires an aqueous stream of acid or base asa starting source or reservoir.

There is a particular need for a pure source of acid or base which canbe generated at selected concentrations solely from an ion exchange bedwithout the necessity of an independent reservoir of an acid or basestarting aqueous stream. There is a further need for such a system whichcan be continuously regenerated. Such need exists in chromatography, andspecifically ion chromatography, as well as other analyticalapplications using acid or base such as in titration, flow injectionanalysis and the like.

SUMMARY OF THE INVENTION

In copending application Ser. No. 08/783,317, filed Jan. 15, 1997, amethod and apparatus is described for generating acid or base in anaqueous stream, such as water alone or in combination with additives(e.g., ones which react with the acid or base or with the sample). Thesystem provides an excellent source of high purity acid or base for useas an eluent for chromatography and, particularly, ion chromatography.The present system is an improvement over the one described in thecopending application.

Referring first to the present system in which a base is generated e.g.for chromatographic analysis of anions, the method includes the stepsof:

-   -   (a) providing a cation source in a cation source reservoir,    -   (b) flowing an aqueous liquid stream through a base generation        chamber separated from the cation source reservoir by a barrier        substantially preventing liquid flow while providing a cation        transport bridge,    -   (c) applying an electric potential between an anode in        electrical communication with said cation source reservoir and a        cathode in electrical communication with the base generation        chamber to electrolytically generate hydroxide ions in the base        generation chamber and to cause cations in the cation source        reservoir to electromigrate toward said first barrier and to be        transported across the barrier toward the cathode to combine        with the transported cations to form cation hydroxide, and    -   (d) removing the cation hydroxide in an aqueous liquid stream as        an effluent from the first base generation chamber.

Suitable cation sources include a salt solution or a cation hydroxidesolution which can be supplied to the cation source reservoir by pumpingfrom a remote reservoir. The solution can be recycled to the remotereservoir. Also, the cation source may comprise a cation exchange bed,e.g., resin particles in a stationary bed or suspended in an aqueousliquid, alone or in combination with the salt solution.

The method may also be used for generating an acid, e.g. for use as aneluent for chromatographic analysis of cations by reversing the chargesof the ion source, the barrier, the electrical potential and any othercharged components of the system.

Another embodiment of the invention is an apparatus for generating anacid or base including:

-   -   (a) an ion source reservoir of either anions or cations,    -   (b) an acid or base generation chamber having inlet and outlet        ports,    -   (c) a first barrier between the ion source reservoir and the        acid or base generation chamber, substantially preventing liquid        flow while providing an ion transport bridge for only ions of        one charge, positive or negative,    -   (d) a first electrode in electrical communication with the ion        source reservoir,    -   (e) a second electrode in electrical communication with the        first acid or base generation chamber, and    -   (f) an aqueous liquid source in fluid communication with the        acid or base generation chamber inlet port.

The apparatus can be used to supply the generated acid or base to achromatography system or any other analytical system which uses a highpurity acid or base.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-8 and 10-12 are schematic representations of apparatus accordingto the present invention.

FIG. 9 is an on-line high pressure gas removal device for use in thepresent invention.

FIGS. 13-29 are graphical representations of experimental results usingthe present base or acid generator system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The system is applicable to the generation of eluent for liquidchromatography forms other than ion chromatography. For example, it isapplicable to liquid chromatography using an ultraviolet (UV) detector.The eluent may be in a form (e.g. salt) other than a pure acid or base.Thus, the term “aqueous stream” includes pure water or water with suchadditives. Also, the terms “eluent comprising a base”, “eluentcomprising an acid”, an “acid” or a “base” mean an aqueous streamincluding acid or base generated according to the invention regardlessof the form it takes on mixing with other reagents present in theaqueous stream. As used herein, the term “cation” excludes hydronium ionand the term “anion” excludes hydroxide ion. The system is alsoapplicable to other non-chromatographic analytical systems which use ahigh purity acid or base.

The copending application uses some of the same principles as thepresent invention and its disclosure is incorporated by reference. Suchdisclosure includes a high purity solution of acid or baseelectrochemically generated by passing deionized water through anelectrically polarized bed of ion exchange resin in the desired ionicform placed between two electrodes. For example, in the generation of aKOH solution, deionized water is pumped through a column packed with acation exchange resin in K⁺ form, and a DC voltage is applied betweenthe anode at the column inlet and the cathode at the column outlet. Theelectrochemical reaction at the anode generates H⁺ ions by splittingwater. Under the influence of the electrical field, H⁺ ionselectromigrate into the resin bed to displace K⁺ ions, which in turnmigrate downstream through the resin bed and combine with OH⁻ ionsgenerated at the cathode to produce KOH. The concentration of KOHgenerated is determined by the electrical current applied and the flowrate of the deionized water through the column. Similarly, a high purityacid (e.g., methanesulfonic acid) solution can be generated using ageneration column containing an anion exchange resin in the desiredionic form.

The acid or base generation column described above is an attractivesource of high purity eluent for ion and liquid chromatography for anumber of reasons. For example, chromatographic separations can beconveniently performed using only deionized water as the carrier. Sinceacid or base is generated on-line, the need of often-tedious, off-linepreparation of eluents can be eliminated. Second, the eluent strength(the concentration of acid or base) can be controlled precisely andconveniently by controlling the electrical current applied to the acidor base generation column and the flow rate. Third, gradientchromatographic separations can be accomplished with current gradientsand a less costly isocratic pump instead of using a more expensivegradient pump. Fourth, the use of an acid or base generation column canimprove the performance of chromatographic methods, since the eluentgenerated on-line can be free of contaminants that are often introducedif it is prepared off-line by conventional means. For example, thepresence of carbonate in hydroxide eluent due to sorption of carbondioxide from air often seriously compromises the performance of an ionchromatography method; this problem will be eliminated by using the highpurity hydroxide eluent generated on-line. Fifth, the reliability of thechromatography pumping system can be improved, the lifetime of pump sealcan be extended significantly since the pump is used to pump deionizedwater instead of more corrosive acid or base solution. These sameadvantages and principles apply to the present invention. In addition,the present invention retains the advantages of the acid or basegeneration column, and provides a significant improvement in thegeneration of high purity acid or base solutions for an extended periodof time for ion and liquid chromatography, and other applications.

The method and apparatus for generation of acid or base according to thepresent invention will first be described to supply eluent, e.g., forion chromatography. Although applicable to anion or cation analysis, thesystem will be described for generation of a base suitable for use as aneluent in the analysis of anions on an ion exchange resin packed bedform. In this instance, the cation exchange bed generates a base such asan alkali metal hydroxide, typically sodium or potassium. For analysisof cations, the eluent generated is an acid such as methanesulfonicacid. The system will first be described for the generation of KOH asthe base.

FIG. 1 schematically illustrates a general form of a large capacity base(KOH) generator form according to the present invention. The apparatusincludes cation (K⁺) ion source reservoir 10. As will be explained inmore detail below, the cation source may be a cation-containing solutionsuch as a salt solution or a cation hydroxide solution. Alternatively,the cation source may be a cation exchange bed including exchangeablecations of the type which form a cation hydroxide. The bed may be formedof ion exchange resin particles in a fixed or stationary bed orsuspended particles in an aqueous liquid. A gas vent may be provided inreservoir 10 to vent oxygen generated therein as described hereinafter.

Base generation chamber 12 is separated from the ion source reservoir 10by a barrier 14, suitably in the form of a charged perm-selectivemembrane described below. Charged barrier 14 substantially preventsliquid flow while providing an ion transport bridge for cations from theion source reservoir 10 to base generation chamber 12. As used herein,the term “barrier” refers to the charged material (e.g. membrane)separating reservoir 10 and chamber 12 which permits ion flow but blocksliquid flow, alone or in combination with an appropriate flow-throughhousing in which the barrier is mounted transverse to flow across theentire flow path.

The charged barrier 14 should be of sufficient thickness to withstandthe pressures in chamber 12. For example, if chamber 12 is on line witha chromatography system, such pressures may be on the order of 1,000 to3,000 psi. When using a membrane as barrier 14, it is suitablyconfigured of circular cross-section within a cylindrical external shortcolumn. Typical dimensions for the membrane are about 4-6 mm diameterand 1-3 mm in length. The barrier can be fabricated by stacking multipledisks of cation membranes together within the cylindrical column.Alternatively, barrier 14 can be prepared from a single ion exchangemembrane of appropriate thickness or a block or rod of appropriate ionexchange material which permits passage of the potassium but not of theliquid.

An anode 16 is disposed in electrical contact with, and preferablywithin, cation source reservoir 10 and a cathode 18 is disposed inelectrical contact with, and preferably within, base generation chamber12. A suitable DC power supply 20 connects the anode and the cathode.Also, there is a continuous electrical path from anode 16 throughbarrier 14 to cathode 18. Aqueous stream 20, suitably deionize water,flows through an inlet port, not shown, in base generation chamber 12.KOH is generated in base generation chamber 12 and flows out of outletport, not shown. A cation exchange resin bed 19 (e.g. in K⁺ form) can bepacked in chamber 12 in contact with barrier 14 and cathode 18 toprovide good electrical contact therebetween. As illustrated, the flowof aqueous stream 20 is toward cathode 18. However, if desired, the flowmay be in the opposite direction.

For the production of pure base (e.g. KOH), high-purity deionized waterfrom source 21 is pumped to generation chamber 12. Water splitting takesplace at both electrodes. The anode reaction in reservoir 10 is asfollows:H₂O−2e ⁻→2H⁺+½O₂  (1)

During this reaction, hydronium ions are produced in reservoir 10 forthe resin form of the invention, the hydronium ions pass into the cationexchange resin by electromigration displacing the exchangeable cations(e.g. K⁺ ions) ahead of them. This displacement takes place along thelength of the bed and the K⁺ ions pass through barrier 14 into chamber12 eventually leading to production of base (KOH) in the flowing aqueousstream in generation chamber 12. The hydroxide ions are produced in thefollowing cathodic reaction.2H₂O+2e ⁻→2OH⁻+H₂  (2)

In one form of reservoir 10, the cation source is a cation-containingsolution, suitably either a salt solution or a cation hydroxide solution(e.g. KOH). If a salt solution is used, it is preferably of a weaklyacidic anion salt such as K₂HPO₄ to bind the hydronium ions produced atthe anode. In this manner, K⁺ is the primary ion passing through barrier14, thereby minimizing the flow of H⁺ ions. The hydronium ion generationin the reservoir provides electrical neutrality to the solution in thereservoir as the K⁺ ions are driven across the barrier.

Another embodiment of the invention is illustrated in FIG. 2. Thisdevice is specifically adapted for use with an ion exchange resin formof cation source in reservoir 10. Because of the similar components inFIGS. 1 and 2, like parts will be designated with like numbers. Theillustrated reservoir 10 is suitably in the form of a solid horizontalhollow cylinder 10 a with inlet and outlet walls 10 b and 10 c,respectively, and packed with cation exchange resin in K⁺ form.Alternative shapes, e.g. rectangular, of reservoir 10 may be used. Anaqueous stream, suitably dionized water, is pumped through an inletport, not shown, into reservoir 10. Similarly, a preferred housing forchamber 12 is a cylindrical column defining a cylindrical chamber. Thus,the terms “chamber” and “column” will be used interchangeably forchamber 12. Anode 16 is illustrated as a perforate disk disposed at theinlet side of reservoir 10 adjacent inlet wall 10 b. Flow-through cationexchange resin bed 24 is suitably of similar ion exchange and flowcharacteristics to a chromatographic separation bed.

A preferred form of ion exchange resin bed in reservoir 10 is a“dual-bed” including a long section 24 a of a strongly acidic cationexchange resin (e.g. a sulfonated resin such as sold under thetrademarks Dowex 50WX8 resin or Dionex ASC resin) in K⁺ form adjacent atthe line X-X to a shorter section of a weakly acidic cation exchangeresin (e.g. a carboxylate resin such as sold under the trademarks DionexCS12A resin or Bio-Rex 70 resin) in K⁺ form downstream at its outletend. As used herein, “weakly acidic” anion means an anion with an aciddissociation constant (pKa) of greater than 3.0 and “strongly acidicanion” means an anion with a pKa less than about 3.0. Preferably thestrongly acidic section 24 a is at least about 10 percent of the lengthor volume of reservoir 10 and more preferably at least about 90 percentof the length or volume. Alternatively, if desired, the entire bed 24may be formed of strongly acidic cation exchange resin.

The dual-bed approach increases the useful capacity of a KOH generatorcolumn. Once H⁺ ions reach the bed of the weakly acidic resin, migrationof H⁺ through the resin bed is significantly slowed down because of itshigher affinity to the weakly acidic functional groups. On the otherhand, the migration of K⁺ ions through the resin bed is notsignificantly reduced.

Therefore, more K⁺ ions are able to reach the cathode to form KOH beforethe arrival of H⁺ ions at the cathode, and thus the useful capacity ofthe KOH generator column is increased. In the dual-bed once H⁺ ionsreach the weakly acidic resin bed, the applied voltage needed tomaintain the constant current will increase due to the development ofthe less conductive protonated zone in the weakly acidic resin bed.

One function of barrier 14 is to permit use of a very large reservoir 10(e.g. 1-2 liters) supplying K⁺ ions to generation chamber 12. This largecapacity reservoir permits a long term supply of K⁺ ions. By way ofexample, a typical KOH generation chamber may have a volume on the orderof less than 100 μL and more typically from 100 μL to 1,000 μL. Suitabledimensions for a cylindrical shape are 4-7 mm ID and 10-50 mm in length.This facilitates use on line in a chromatography system. In contrast,reservoir 10 may be many times larger than the volume of the generationchamber 12. For example, the ratio between reservoir 10 and chamber 12may be at least 5:1 to 10:1 or 20:1 or even higher.

Another function of barrier 14 is that it provides a high pressurephysical barrier that insulates the relatively low pressure K⁺ ionsupply reservoir 10 from the generation chamber 12 which is ofsubstantially high pressure when it is on line with a high pressurechromatography system. For example, even a very low pressurechromatography system would be pressurized to at least about 50 psi.Assuming the reservoir's atmospheric pressure (14.7 psi) the pressuremaintained in the base generation chamber 12 is at least about threetimes the pressure maintained in reservoir 10. This isolation isparticularly useful when that pressure ratio is at least about 2:1 andis even more so when the ratio is much higher, for example at leastabout 5:1 to at least about 10:1 to 100:1 or higher.

Because it is operated under low pressure, a large K⁺ ion supply columncan be prepared and operated safely without demanding pressureconstraint. A large K⁺ ion supply column can contain a sufficient amountof cation exchange resin in K⁺ form to generate KOH over an extendedperiod of time. For example, a 10-cm ID×20-cm length K⁺ ion supplycolumn has an internal volume of 1570 mL and can contain 2670 meq of K⁺ions (calculated using the resin capacity of 1.7 meq/mL). If the KOHgenerator column is used to generate 20 mM KOH at 1.0 mL/min, itstheoretical capacity is 2225 hours, and an actual useful time isexpected to be more than 1300 hours, assuming 60 percent of the total K⁺ion capacity is ultimately utilized for the generation of KOH.

To step down from the large volume reservoir 10 to the smaller size basegeneration chamber 12, an adapter section in the form of hollowcylindrical column 26 packed with cation exchange resin 28 may bedisposed in open communication with column 10 through an opening in theend wall 10 c of reservoir 10. Barrier 14 is disposed between cylinder26 and generation chamber 12. A suitable configuration of barrier 14 isa hollow cylinder transverse to cylinder 26 with a barrier disk (e.g.permselective membrane) across the flow path therebetween. Generationchamber 12 also is suitably is in the form of a hollow cylinder.

Barrier 14 is suitably in the form of a stack of cation exchangemembranes or a plug which prevents any significant liquid flow butpermits transport of the K⁺ ions into chamber 12. A suitable form ofmembrane is supplied by Membrane International of Glenrock, N.J.(designated CMI-7000 cation exchange membrane). As illustrated, cathode18 is a porous disk disposed adjacent to and coextensive with the endwall at the exit of chamber 14. As in the embodiment of FIG. 1, water issupplied to an inlet port of chamber 12. The KOH generated near cathode18 exits from the outlet of chamber 12. This is advantageous as the H₂gas generated at the cathode is readily swept out of chamber 12.

Anode 16 and cathode 18 disposed in reservoir 10 and generation chamber12, respectively, can take the different forms such as porous disks,frits, rings, screens, sheets, and probes so long as they provide goodcontact (preferably direct contact) with the ion source or ion exchangeresin. For example, the anode is preferably in direct contact with theion exchange resin, if used, or with the solution in the reservoir if noion exchange resin is used. Similarly, the cathode should be in directcontact with the ion exchange resin when used in the generation chamber.The electrode may also be formed by crumpling and forming a length offine platinum wire to form a roughly disk-shaped object that allows easyflow through the structure. The electrodes are preferably made of inertmaterial, such as platinum. In the embodiments described above, it ispreferable that the electrodes be placed in a region near the outlet ofgeneration chamber 12, although other locations may be used as well.

In another form of the electrodes, not shown, a thin inert electricallyconductive screen is wrapped partially or totally around a bed of ionexchange resin in chamber 12 in a case-like configuration. Thiselectrode design provides good contact between the cation exchange resinand the electrode surface, thus lowering the device operating voltage.Thus, higher currents can be applied to generate higher concentrationswithout being limited by possible excessive heating.

In general, the method of the present invention using the embodiment ofFIG. 2 is performed as follows. The cation source is provided by thecombination of cation exchange resin 24 in reservoir 12 and cationexchange resin 28 in column 26. The H⁺ ion formed near anode 12 drivesthe K⁺ ions through the resin until they transport across barrier 14.The H⁺ ions produce electrical neutrality to reservoir 10. The K⁺ ionstravel across barrier 14 into chamber 12 towards cathode 18 and combineswith the hydroxide ions formed at the cathode to form KOH. The aqueousstream flowing through base generation chamber 12 carries the KOH insolution for subsequent use in the analytical system.

When using a packed ion exchange bed in reservoir 10 or generationchamber 12, the higher the cross-linking of a resin the higher itscapacity (expressed as milliequivalents per ml. of column); therefore,higher cross-linked resins give more compact generators. This isdesirable. However, the higher the cross-linking of a resin, the less itdeforms when packed in a column. Some deformation is desirable in thatit improves the area of contact between resin beads thus lowering theelectrical resistance of the packed bed. Lower resistance means that aparticular level of current may be attained at a lower applied voltage;this, in turn, leads to less heating of the bed while carrying current,a desirable feature.

Bead deformation is favored by lowering the degree of cross linking.But, resin of very low cross-linking (say 1 to 2%) is so deformable thatat certain flow rates the deformation can lead to undesirably highpressure across the bed. In summary, a wide range of cross-linking canbe used. Resins of moderate cross-linkage are to be preferred, typicallyin the range of 4 to 16% divinyl benzene for styrene divinyl benzenepolymer beads.

Other forms of ion exchange beds can be used such as a porous continuousstructure with sufficient porosity to permit flow of an aqueous streamat a sufficient rate for use as an eluent for chromatography withoutundue pressure drop and with sufficient ion exchange capacity to form aconductive bridge of cations or anions between the electrodes. One formof structure is a porous matrix or a sponge-like material with aporosity of about 10 to 50% permitting a flow rate of about 0.1 to 3ml/min without excessive pressure drop. Another suitable form is a rollof ion exchange film (e.g. in a configuration of such a roll on aspindle disposed parallel to liquid flow). Electrodes would be placed ateach end of the roll which could be textured to provide an adequate voidchannel.

The aqueous stream flowing through chamber 12 may be high-puritydeionized water. However, for use in some forms of chromatography, itmay be desirable to modify the source with an additive which reacts withthe base generated in electrode chamber 12 to produce eluents of varyingpotency. For the production of base, some well known additives include asource of carbonic acid, phenol, cyanophenol, and the like. (For theproduction of acid, such additives include m-phenylene diamine,pyridine, lysine and amino propionic acid.)

It is preferable to control the concentration of base produced in basegeneration chamber 12. To do so, the current, directly related toconcentration, is controlled. A feed-back loop may be provided to assuresufficient voltage to deliver the predetermined current. Thus, thecurrent is monitored when the resistance changes, and the potential iscorrespondingly changed by the feed-back loop. Therefore, the voltage isa slave to the reading of the current. Thus, it is preferable to supplya variable output potential system of this type (e.g., sold under thedesignation Electrophoresis Power Supply EPS 600 by Pharmacia Biotechand Model 220 Programmable Current Source by Keithley).

The current (voltage) requirements of a generator depend on (a) theeluent strength required; (b) the diameter of the column; (c) the lengthof the column; (d) the electrical resistance of the resin; and (e) theflow rate of the aqueous phase.

FIG. 3 illustrates another embodiment of the invention. In thisinstance, no ion exchange resin is used in reservoir 10. Instead, asolution of a potassium salt such as K₂HPO₄ is employed. Alternatively,for specific applications, KOH may be used. The potassium salt solutionmay be used in combination with a cation exchange resin in K⁺ formeither in a fixed resin bed or in a bed in which the resin particles aresuspended in the solution. The concentration of K⁺ ions in solution ispreferable about 1 to 2 M or higher so that there is a sufficient amountof K⁺ ions for the generation of KOH over an extended time. However, ifdesired, the potassium salt solution containing K⁺ ions at lowerconcentrations (e.g. 0.1 to 0.5 M) can be used for specificapplications. It is preferable that the anion of the potassium salt notbe oxidized by the anode. It is preferable to use a potassium weaklyacidic anion (e.g., HPO₄ ²⁻ or CO₃ ²⁻) with an acid dissociationconstant (pK_(a)) of 5 or higher so that the concentration of free H⁺ions in the solution is kept lower than 0.1 mM. H⁺ ions, like K⁺ ions,can migrate across barrier 14 into generation chamber 12. If such H⁺migration occurs in significant amounts, the direct linear relationshipbetween the applied current and the concentration of KOH generated canbe lost because H⁺ ions can be combined with OH⁻ ions generated at thecathode to form water and thus the performance of the system can becompromised. By using the K₂HPO₄ salt, the following reaction occursusing H⁺ generated at anode 16 in equation (1) above.2H⁺+2HPO₄ ²⁻=2H₂PO₄ ⁻  (3)

As in the embodiments of FIGS. 1 and 2, an aqueous stream is pumpedthrough the generation chamber at 12 and a DC voltage is applied betweenanode 16 and cathode 18. K⁺ ions migrate from reservoir 10 intogeneration chamber 12 through barrier 14 in the same manner describedabove. Also, as set out above, barrier 14 provides a high-pressurephysical barrier that prevents liquid leakage and diffusion of any ionsfrom reservoir 10 into generation chamber 12.

One advantage of this embodiment in which a solution without resin isused in reservoir 10 is that the potassium salt (e.g., K₂HPO₄) is a lessexpensive source of K⁺ ion than ion exchange resin with exchangeable K⁺ions. Also, it is easier to replenish the reservoir with a fresh sourceof potassium salt. By way of example, in the embodiment of FIG. 3 usinga one liter reservoir filled with 2.0 M K₂HPO₄ as a theoretical capacityof 4,000 meq K⁺ ions to generate 20 mM KOH at 1.0 mL/min, the devicewill have a useful lifetime of 2500 hours, assuming a 75 percentconsumption of K⁺ ions in its K⁺ ion supply reservoir before replacingthe salt solution.

FIG. 4 illustrates a flow-through strongly acidic cation exchange resinbed 30 in K⁺ form disposed in reservoir 10. Anode 12 is suitably in theform of a perforated platinum electrode at its outlet and adjacent anoutlet port, not shown. Generation chamber 12 is separated fromreservoir 10 by barrier 14 of the type described above. In thisinstance, cation solution in the form of the potassium salt (e.g., 2.0 MK₂HPO₄) is continuously pumped by a pump 34 to a reservoir 10 at adesired rate (e.g. about 0.1 to 2.0 mL/min). The same principlesdescribed above with respect to concentration of the potassium salt andthe type of salt applied to this embodiment as well. Similarly, the sameflows and reactions occur in generator 12.

Continuous pumping of the potassium salt solution leads to a continuoussupply of K⁺ ions until the solution of salt in the remote reservoir isconsumed.

In one embodiment illustrated in FIG. 4, the potassium salt solution isrecycled in recycle line 36 from the outlet of reservoir 10 to the inletof remote reservoir 32. The system can be operated until theconcentration of K⁺ ions in remote reservoir 32 has been decreased to alevel insufficient to consistently generate KOH at the desiredconcentration. Then the device can be replenished by replacing thepotassium salt solution in the remote reservoir 32. Alternatively, inthe non-recycle mode, the solution exiting reservoir 10 flows to wasteas illustrated by dotted line 38. The flow rate of the potassium saltsolution can be slightly adjusted (e.g., about 0.005 to 0.050 mL/min) toprovide a sufficient supply of K⁺ ions to generate KOH at the desiredconcentration. Similarly, the device is replenished by filling theremote reservoir with potassium salt solution when the concentration hasdropped below the desired level.

In another embodiment of the invention, not shown, ion exchange resin 30may be eliminated from reservoir 10 so reservoir 10 is filled with saltsolution flowing from a remote reservoir 32. Otherwise the system isidentical to the one described above.

Referring to FIG. 5, another embodiment of the invention is illustratedincluding multiple generation chambers 12 a, 12 b, and 12 c connected inseries, each one including its own cathodes 18 a, 18 b, and 18 c.Generation chambers 12 a, 12 b, and 12 c are connected to reservoir 10by barriers 14 a, 14 b, and 14 c as described above. The difference isthat there are smaller generation chambers and smaller barriers. By wayof example, if each generation chamber is applied with a current of 80mA to generate 25 mM of KOH at 2.0 mL/min the KOH generator with threegeneration chambers is capable of producing about 75 mM of KOH at 2.0mL/min. Additional KOH generation chambers may also be employed. Anadvantage of using two or more generation chambers is that the operatingvoltage of the system may be lowered because the applied current used togenerate KOHs distributed among the generation chambers. Thus highercurrents may be applied to generate the base of higher concentrationswithout being limited by potentially excessive heating.

In another embodiment, not shown, two or more cathodes may be disposedin a generation chamber 12, preferably spaced along the length of thechamber in the direction of aqueous liquid flow, e.g. near the inlet andoutlet. This can serve to lower the electrical resistance of the chamberand thus the operating voltage of the system.

Referring to FIG. 6, another embodiment of the invention is illustratedusing a single generation chamber 12 and two barriers 14 a and 14 binterconnecting chamber 12 and reservoir 10. Use of multiple barrierscan reduce the device operating voltage. Therefore the generationchamber 12 can be supplied with higher currents to generate KOH athigher concentrations without being limited by potentially excessiveheating. Another advantage in the use of multiple barriers is thatflexible membranes of smaller areas have better resistance to burstingthan larger area membranes.

Referring to FIG. 7, use of the KOH generator of the present inventionis schematically illustrated on-line in an ion chromatography or liquidchromatography system. Water from source 40 is pumped by pump 42 throughthe generation chamber of the large capacity KOH generator 44 with ananode in the cation source reservoir and a cathode in the generationchamber connected to a power supply 45, as described above. Generator 44is on-line with a conventional simplified ion chromatography system.Pump 42 is a conventional chromatography pump which pumps the KOH outputfrom generator 44 through sample injection valve 48 into chromatographicseparator 50 packed with a chromatographic separation medium, typicallyan ion exchange resin packed bed column. Alternatively, other forms ofseparation medium may be used such as porous hydrophobic chromatographicresin with essentially no permanently attached ion exchange sites.

In ion chromatography, the effluent from the separation column 50 flowsthrough suppressor 52 serving to suppress the conductivity of the baseand the effluent from separator 50, but not the conductivity of the ionsinjected through sample injector 48. Then, the effluent from suppressor52 is directed through a flow through detector 54, e.g. a conductivitydetector, for detecting the resolved ions in the effluent fromsuppressor 52. A suitable data system, not shown as provided in the formof a conventional conductivity detector for measuring the suppressoreffluent in the conductivity cell in which the presence of an ionicspecies produces an electrical signal proportional to its concentration.With the exception of generator 44, such ion chromatography systems arewell known as illustrated in U.S. Pat. Nos. 3,897,213; 3,920,397;3,925,019; and 3,956,559 incorporated herein by reference.

Other forms of detectors 54 may also be employed and the suppressor maybe eliminated. Such other forms of detection include UV, fluorescenceand electrochemical.

In the large capacity KOH generator, electrolysis reactions producehydrogen and oxygen gases. When used in a chromatography system, thehydrogen gas, along with the KOH solution, is carried forward into thechromatographic flow path. If hydrogen gas is produced in a significantvolume relative to the liquid flow, its presence can be detrimental tothe downstream chromatography process. The potential problem of hydrogengas can be eliminated by application of Boyle's law. A flow restrictorcan be placed after the detector flow cell to elevate the pressure ofthe entire chromatography system. Under high pressure (e.g., 1000 psi orhigher pressures), hydrogen gas is compressed to an insignificant volumecompared to the eluent flow so that it does not interfere with thedownstream chromatography process. This approach requires the use of adetector flow cell capable of withstanding a pressure of 1000 psi ormore. In an ion chromatography system using suppressed conductivitydetection, the above approach also requires the use of a suppressor thatis capable of withstanding a pressure of 1000 psi or more. The necessarypressure to accomplish this depends on the volume of gasses produced.However, for a typical system, a pressure of at least 250 to 500 psi issufficient. One mode of elevating the pressure is to connect a flowrestrictor 56 such as a fine bore coiled tubing downstream of thedetector (e.g. three meters of 0.005 in I.D.). This elevates thepressure throughout the chromatography system upstream of the detector.

Another approach to eliminate the potential problem associated withhydrogen gas is to use an on-line pressure gas removal device to removehydrogen gas from the KOH solution. FIG. 8 illustrates a schematicoutline of an ion chromatography system employing a large capacity KOHgenerator and an on-line high pressure gas removal device 60 instead offlow restrictor 56 in FIG. 7. In this implementation, a high pressuregas removal device 60 is placed downstream of the outlet of the largecapacity KOH generator 44, suitably between it and sample injector 48.Hydrogen gas is effectively removed from the KOH eluent before itreaches the sample injector of the chromatography system so that thedownstream chromatographic process is not affected. One advantage ofthis system is that a conventional detector flow cell and ionchromatography suppressor can be used.

One preferred embodiment of the on-line high pressure gas removal deviceis shown in FIG. 9. In this embodiment, gas permeable polymeric tubing62 is used to remove hydrogen gas in the KOH product solution under highpressure. Aqueous solution 67 flows in an annular space 64 outside ofthe gas permeable tubing 62 defined between tubing 64 and protectivetubing 66. The released hydrogen gas is removed from the device by inthe flowing aqueous liquid stream in space 64 which also serves toprevent absorption of carbon dioxide from the ambient air into the KOHproduct stream. One source of the aqueous liquid in space 64 is thedetector effluent.

Preferably, the polymeric tubing 62 is inert and has high burst pressureand high gas permeability. The inner volume of the gas permeable tubingshould be small so that it does not have large dead volume and thus doesnot compromise the gradient performance of the large capacity eluentgenerator. It is preferred to use a gas permeable tubing with insidediameter less than 0.015 inch so that the gas removal device has lowdead volume and high burst pressure.

The polymeric tubing prepared from a number of polymers includingpolymethylpentene, polypropylene, and fluoropolymers such as PTFE, ETFE,PFA, and FEP is gas permeable under high pressure and may be used as thegas removal tubing for the eluent generator.

The on-line high pressure gas removal device shown in FIG. 8 can also beused to remove oxygen gas generated along with the acid solution in alarge capacity acid generator.

In another embodiment of the invention, not shown, the system of FIG. 7can be used in gradient ion or liquid chromatography where eluentcomponents in addition to KOH are required. A gradient pump, e.g. aDionex GP-40 pump type, can be used to deliver a prescribed mixture ofone or more eluent components from separate reservoirs to the highpressure KOH generation column. The eluent is modified with KOH which isgenerated on-line at the exit end of the KOH generation column. Theconcentration of KOH in the final eluent delivered to the separationcolumn can be controlled by controlling the applied current to the largecapacity KOH generator. The gradient system using the large capacity KOHgenerator is especially beneficial to applications that require the useof highly pure base hydroxide solution.

Referring to FIG. 10, another form of the present invention isillustrated. Here reservoir 10 includes a solution of cation saltsolution (e.g. one liter of K₂HPO₄ at 2 M concentration). Barrier 14extends substantially along the entire length of the mating sidegeneration chamber 16 in open communication with the interior of thechamber. Cathode 18 is in the form of a perforated platinum cathodewhich extends along the flow path of the aqueous stream through chamber12 in direct contact with beds of ion exchange resin 19 in K⁺ form onboth sides of cathode 18. Water flows through an inlet port, not shown,on the upstream side of the chamber. The KOH produced in chamber 12exits at an outlet port, not shown, at the downstream side of thechamber. The perforated platinum cathode is in the form of a screensuitably extending along the entire length of resin bed and isperforated to permit passage of solution through the cathode to ensurean efficient removal of KOH generated.

Another form of generation chamber 12 is illustrated in FIG. 11. Thisembodiment differs from that of FIG. 9 in the use of a cation exchangescreen 70 in contact with perforated cathode 18 on one side and withbarrier 14 on the other side. The electrical path between anode 16 andcathode 18 extends through barrier 14, cation exchange screen 10 andperforated cathode 18. The aqueous stream flows through the chamber 12inlet port, through perforated cathode 18 into cation exchange screen 70where it flows adjacent to the cathode and out the chamber 12 outlet onthe downstream side of screen 70.

In another embodiment of the generation chamber, not shown, the onlystructural eluent within chamber 12 is cathode 18 in the form of aperforated platinum electrode screen in direct contact with barrier 14.The aqueous stream flows through the perforated platinum cathode screen.The screen uses openings of a size suitably on the order of 50-100 μm topermit the flow of the aqueous stream through the platinum screenwithout undue pressure drops. A suitable screen has a size of 1 to 5cm².

Another embodiment of the base generation chamber design is illustratedin FIG. 12. As in the embodiment of FIG. 10, barrier 14 extends alongthe entire length of chamber 12. In this instance, the perforatedplatinum cathode 18 is sandwiched between non-charged screens 72 and 74suitably formed of a non-charged polymer such as a polypropylene whichforms the fluid pathway in the generation chamber 12. Screens 72 and 74may be of the same size as the screen cathode in the embodiment of FIG.11. An inert lead, e.g. platinum wire 76, provides electrical contactwith platinum cathode 18 and in direct contact with barrier 14. Upon theapplication of electrical current a small amount of KOH is formed insitu. The KOH serves as the ion transport bridge between barrier 12 andplatinum electrode 18. Screens 72 and 74 have sufficient porosity topermit the flow of water through the screen without undue pressure drop.

The system of FIG. 12 can be operated by first filling chamber 12 withKOH solution prepared externally which serves as the ion transportbridge between barrier 14 and cathode 18. Then current is applied. Goodcontact between the perforated disk-cathode 18 and barrier 14 may bemaintained by pressing one against the other. The electrode can extendacross all or part of the aqueous liquid flow path through the chamber12 to permit intimate contact with the flowing aqueous stream.

Other embodiments of the interior configuration of the base generationchamber may be employed so long as there is sufficient electrical pathbetween the anode and the cathode to permit the cations to transportacross the barrier and with the aqueous stream flowing through thechamber to permit the efficient generation of KOH. It has been foundthat systems in which the cathode and a barrier in the form of a chargedmembrane extends substantially along the entire flow path of the aqueousstream through the base generation chamber is very efficient.

The system has been described with respect to generating a base andspecifically KOH. However, the system is also applicable to thegeneration of an acid by reversal of the polarity of the ion exchangebeds, barrier and the electrodes. In this instance, anion exchange beds,rather than cation exchange beds are employed. Also the barriers are ofa type which pass anions but not cations and block the flow of liquid.Suitable barriers for use in the production of acid can be prepared froma single or multiple ion exchange membrane of appropriate thickness or ablock or rod of ion exchange material. A suitable form of membrane issupplied by Membrane International of Glen Rock, N.J. (designatedAMI-7000 anion exchange membrane).

The cations or anions for use as the source in reservoir 10 must also besufficiently water soluble in base or acid form to be used at thedesired concentrations. Suitable cations are metals, preferably alkalimetals such as sodium, potassium, lithium and cesium. Known packing forhigh capacity ion exchange resin beds provide such cations or anion foruse in the embodiment where resin is used as the source of cations oranions. Typically, the resin support particles would be in the potassiumor sodium form. Potassium is a particularly effective exchangeablecation because of its high conductance. Suitable other cations aretetramethyl ammonium and tetraethyl ammonium. Analogously, suitableexchangeable anions for cation analysis include chloride, sulfate andmethane sulfonate.

Using the concept described above, a large capacity acid generator canalso be implemented. For example, a large capacity methanesulfonic acid(MSA) generator employing a CH₃SO₃ ⁻ ion supply reservoir is describedhere as an example. MSA generation chamber 12 is packed with a stronglybasic anion exchange resin in CH₃SO₃ ⁻ form and equipped with a Ptscreen electrode (anode) which is in direct contact with the anionexchange resin. The MSA generation chamber 12 is connected to the CH₃SO₃⁻ ion supply reservoir 10 using one or more anion ion exchange barriersof the same general type as barrier 14. Barrier 14 permits the passageof CH₃SO₃ ⁻ ions from the supply reservoir into the resin bed in the MSAgeneration column, while precluding the passage of cations from theCH₃SO₃ ⁻ ion supply reservoir into the MSA generation column. Barrier 14also serves the role of a high pressure physical barrier that insulatesthe low pressure CH₃SO₃ ⁻ ion supply compartment from the high pressureMSA generation chamber 12.

Analogous to the cation-source reservoir, the anion-source (CH₃SO₃ ⁻)reservoir 10 is equipped with a cathode and a gas vent hole. Thereservoir (1 to 2 liters in volume) is filled with a solution of a MSAsalt such as NH₄CH₃SO₃. The concentration of CH₃SO₃ ⁻ ions in thesolution is preferably 1 to 2 M or higher so that there is a sufficientamount of CH₃SO₃ ⁻ ions in the CH₃SO₃ ⁻ ion supply reservoir for thegeneration of MSA over an extended period of time; however, the MSA saltsolution containing CH₃SO₃ ⁻ ions at lower concentrations can be used.It is preferred that the cation of the MSA salt used can not be reducedby the cathode in the CH₃SO₃ ⁻ ion supply reservoir. It is alsopreferred to use a “weakly basic cation” (e.g., NH₄ ⁺) defined to have abase dissociation constant (pK_(b)) of 4.5 or higher so that theconcentration of free OH⁻ ions in the solution is kept lower than 0.1mM. A “strongly basic cation” is defined to have a base dissociationconstant (pK_(b)) of less than 4.5. OH⁻ ions, like CH₃SO₃ ⁻ ions, canmigrate across the anion exchange connector into the MSA generationcolumn. If OH⁻ ions migrate across the anion exchange connector into theMSA generation column in significant amounts, the direct linearrelationship between the applied current and the concentration of MSAgenerated is lost because OH⁻ ions can combine with H⁺ ions generated atthe anode to form water, and thus the performance of the MSA generatoris compromised.

To operate the large capacity MSA system, deionized water is pumpedthrough the MSA generation chamber 12, and a DC voltage is appliedbetween the anode and cathode 18. Under the applied field, theelectrolysis of water occurs at the anode and cathode. Water is reducedto form OH⁻ ions and hydrogen at the cathode:2H₂O+2e ⁻→2OH⁻+H₂↑  (4)and oxidized to form H+ ions and oxygen at the anode:H₂O+2e ⁻→2H⁺+½O₂↑  (5)CH₃SO₃ ⁻ ions migrate through barrier 14 into the resin bed in the MSAgeneration chamber 12, and eventually combine with H⁺ ions generated atthe anode to produce a MSA solution suitable for use as a high purityeluent for ion or liquid chromatography.

The large capacity acid or base generator can also be implemented togenerate high purity ion pairing reagents such as octanesulfonic acid(OSA) and tetrabutylammonium hydroxide (TBAOH) for use as eluents inmobile phase ion chromatography (MPIC) or reversed-phase ion pairchromatography (RPIPC).

Although much of the above discussion relates to use of the generatedbase or acid in ion and liquid chromatography, such use can also beapplied to other areas such as titration, flow injection analysis andpost-column reactors.

Specifically the generated base can be used in combination with (a)conventional titration analyses, e.g. described in Douglas A. Skoog andDonald M. West, Fundamentals of Analytical Chemistry, 4th Edition,Saunders College Publishing, San Francisco, 1982, Chapter 8 Theory ofNeutralization, p. 195 or Douglas A. Skoog, Principles of InstrumentalAnalysis, 3rd Edition, Saunders College Publishing, San Francisco, 1985,Chapter 20 Potentiometric Methods, p. 638; (b) flow injection analysis,e.g., described in Theory and Automation, Skoog, Chapter 29, p. 858-859;and (c) post-column reactors, e.g. described in Paul R. Haddad and PeterE. Jackson, Ion Chromatography, Elsevier, N.Y., 1988, p. 387 and R. W.Frei Editor and K. Zech, Selective Sample Handling and Detection inHigh-Performance Liquid Chromatography, Elsevier, N.Y., 1988, p. 396.

The following examples are provided in order to further illustrate thepresent invention.

EXAMPLE 1 Generation of KOH Using a KOH Generator Employing a LargeCapacity K⁺ Ion Supply Reservoir (as Illustrated in FIG. 2)

A large capacity KOH generator consisting of a K⁺ ion supply reservoir10 in the form of column (18-mm ID×185-mm length) and a KOH generationchamber in the form of column 12 (4-mm ID×30-mm length) was constructed.

The KOH generation chamber was packed with an 18-μm, 8% cross-linksulfonated styrene/divinyl benzene resin in K⁺ form. The K⁺ ion supplycolumn consisted of a 175 mm length bed of an 18 μm, 8% cross-linksulfonated styrene/divinyl benzene resin in K⁺ form and a 10 mm lengthbed of a 50 μm polyacrylate resin in K⁺ form. The device was testedunder an applied current of 30 mA and a carrier flow rate of 1.0 mL/minfor 48 hours. The conductance of the KOH solution generated and theoperating voltage of the KOH generator were monitored over the testingperiod. The exhaustion profile (the conductance of the KOH solutiongenerated vs. time) and the operating voltage data are shown on FIG. 13.The device produced a constant output of KOH (18.7 mM KOH at the carrierflow rate of 1.0 mL/min) for 44.4 hours, or a useful capacity of 49.7meq. After 44.4 hours of operation, the operating voltage increased to275 V (the operating voltage limit of the power supply used in theexperiment) due to the development of a less conductive neutralized zonein the weakly acidic carboxylated resin bed inside the K⁺ ion supplycolumn, and decreases in the operating current and concentration of KOHgenerated were observed. These results indicate the feasibility of usingthe large capacity KOH generator employing the large K⁺ ion supplycolumn to generate the KOH solution over an extended period of time.

EXAMPLE 2 Generation of KOH Using a Large Capacity KOH GeneratorEmploying a Flow-Through K⁺ Ions Supply Column (as Illustrated in FIG.4)

A large capacity KOH generator employing the flow-through K⁺ ion supplycolumn was constructed to evaluate this embodiment of the invention(FIG. 4). Both the flow-through K⁺ ion source reservoir 10 in the formof column (4-mm ID×25-mm length) and the KOH generation chamber (4-mmID×25-mm length) were packed with an 18 μm, 8% cross-link sulfonatedstyrene/divinyl benzene resin in K⁺ form and equipped with porous Pt fitelectrodes at their outlets. A 100-mM KCI solution in a remote reservoirwas pumped continuously through the flow-through K⁺ ion supply column ata flow rate of 1.0 mL/min. The large capacity KOH generator was testedunder applied currents of 10.5, 21, and 30.5 mA for about 23 hours. Theoperating voltage ranged from 40 to 60 V during the experiment. FIG. 14shows the conductance profiles of the KOH solutions generated at acarrier flow rate of 1.0 mL/min and applied currents of 10.5, 21, and30.5 mA. The concentration of KOH generated was directly proportional tothe applied current. The results indicate that it is feasible to use alarge capacity KOH generator employing a flow-through K⁺ ion supplycolumn to generate the KOH solution over an extended period of time.

EXAMPLE 3 Generation of KOH Using a Large Capacity Generator Employing aK⁺ Ion Supply Reservoir (as illustrated in FIG. 3)

A large capacity KOH generator employing a K⁺ ion source reservoir 10was constructed to evaluate this preferred embodiment of the invention(FIG. 3). The KOH generation chamber (5.2-mm ID×37-mm length) was packedwith an 18-μm, 8% cross-link sulfonated styrene/divinyl benzene resin inK⁺ form and equipped with a porous Pt frit electrode at its outlet. TheK⁺ ion source reservoir 10 was filled with a 2.0 M K₂HPO₄ solution. Thelarge capacity KOH generator was operated continuously under a constantcurrent of 30 mA and a carrier flow rate of 1.0 mL/min for a total of832 hours. The operating voltage was about 60 V during the test. The KOHsolutions generated using the device were periodically collected andtitrated using a 10-mM nitric acid standard to determine theconcentration of KOH generated. FIG. 15 shows the determinedconcentration of KOH in the solutions collected. Over the period of 744hours, the average determined KOH concentration was 17.7 mM (n=18 andRSD=2.2%), corresponding to 95% of the theoretical concentration of 18.7mM. The results indicate that it is feasible to use a large capacity KOHgenerator employing a large capacity K⁺ ion supply reservoir to generatethe KOH solution over an extended period of time.

EXAMPLE 4 Generation of KOH Using a Large Capacity Generator Employing aK⁺ Ion Supply Reservoir and Three KOH Generation Chambers (asIllustrated in FIG. 5)

A large capacity KOH generator employing a K⁺ ion supply reservoir andthree KOH generation chambers, as illustrated in FIG. 5, wasconstructed. Each KOH generation chamber (5.2-mm ID×10-mm length) waspacked with an 18-μm, 8% cross-link sulfonated styrene/divinyl benzeneresin in K⁺ form and equipped with a porous Pt frit electrode at itsoutlet. The K⁺ ion supply reservoir was filled with a 2.0 M K₂HPO₄solution. The large capacity KOH generator was used to generate KOHsolutions under applied currents ranging from 10 to 160 mA and carrierflow rates of 1.0 or 2.0 mL/min. The operating voltage for the KOHgenerator was 45 V when an applied current of 160 mA was maintained togenerate 50 mM KOH at 2.0 mL/minute.

The concentrations of KOH generated at different applied currents usingthe KOH generator were determined by titration using a 10-mM nitric acidstandard. The results are summarized in Table 1. In this KOH generator,the KOH solution generated in the first KOH generation chamber flowsthrough the second and third KOH generation chambers. The presence ofKOH solution in the second and third KOH generation chambers did notaffect the KOH generation in the second and third chamber. The percentelectrolytic yield of this KOH generator was very close to thetheoretical limit, ranging from 96.8 percent at 10 mA to 99.0 percent at100 mA, as shown in Table 1. There was also excellent correlation(R²=0.9998) between the applied current and the determined concentrationof KOH generated (FIG. 16).

TABLE 1 Calculated and Determined Concentrations of KOH Generated Usinga Large Capacity KOH Generator with Three KOH Generation ChambersCalculated Determined Percent Percent Applied Flow rate, Concentration,Concentration^(a), mM Yield^(b) RSD Current mL/min mM (n = 3) (n = 3) (n= 3)  10 mA 2.0 3.1 3.0 96.8 0.2  50 mA 2.0 15.5 15.1 97.4 0.4 100 mA2.0 31.1 30.8 99.0 0.9 100 mA 1.0 62.2 61.2 98.4 0.5 30 mA + 10 mM 2.019.3 19.2 98.9 0.9 NaOH 60 mA + 10 mM 2.0 28.7 28.4 98.4 1.3 NaOH^(a)The number of determinations was three. ^(b)Percent yield wascalculated using the following definition: Percent yield = (Determinedconcentration − Calculated concentration)/Calculated concentration * 100

The above results indicate that connecting multiple KOH generationchambers in series is a viable approach to boost the concentration ofKOH generated. The results also demonstrate that KOH at relatively highconcentrations can be accurately generated using a large capacity KOHgenerator with multiple KOH generation chambers without being limited byexcessive heating.

EXAMPLE 5 Evaluation of a Large Capacity KOH Generator Employing a KOHGeneration Chamber with Multiple Ion Exchange Connectors (as Illustratedin FIG. 6)

A large capacity KOH generator employing a K⁺ ion source reservoir and aKOH generation chamber in the form of column with two multiple ionexchange connectors, as illustrated in FIG. 7, was constructed. The K⁺ion supply reservoir was filled with a 2.0 M K₂HPO₄ solution. The KOHgeneration chamber 12 in the form of column ((5.2-mm ID)×10-mm length)was packed with an 18-μm, 8% cross-link sulfonated styrene/divinylbenzene resin in K⁺ form and equipped with a porous Pt frit electrode atits outlet. The KOH generation column was connected to the K⁺ ion supplyreservoir using either one or two ion exchange connectors (each with a 5mm in contact diameter) during the experiment. The applied current wasvaried from 10 to 90 mA and the operating voltage was monitored. Thecarrier flow rate was maintained at 2.0 mL/minute.

The dependence of the operating voltage on the applied currentdetermined for the KOH generator is shown in FIG. 17. For a givenapplied current, the operating voltages required for the generator usingtwo ion exchange connectors were about 30 percent lower than thoserequired for the generator using one ion exchange connector. The use ofmultiple ion exchange connectors in a single KOH generation columnclearly increases the pathway for the transport of K⁺ ions from the K⁺ion supply reservoir into the KOH generation column and thus reduces thedevice operating voltage. The results suggest that the use of multipleion exchange connectors in a single KOH generation column is a viableapproach to facilitate the generation of KOH at relatively highconcentrations.

EXAMPLE 6 Evaluation of Different Cathode Configurations for the LargeCapacity KOH Generator

A large capacity KOH generator employing a K⁺ ion source reservoir, asillustrated in FIG. 3, was constructed. The K⁺ ion supply reservoir wasfilled with a 2.0 M K₂HPO₄ solution. The KOH generation chamber in theform of column (5.2-mm ID×10-mm length) was packed with an 18 μm, 8%cross-link sulfonated styrene/divinyl benzene resin in K⁺ form. The KOHgeneration column was connected to the K⁺ ion supply reservoir using oneion exchange connector (5 mm in contact diameter). Three cathodeconfigurations were tested for the KOH generation column: one porous Ptfrit (4 mm diameter) placed at the outlet of the generation column, twoporous Pt frits (4 mm diameter) placed at the inlet and outlet of thegeneration column, and a Pt screen that is formed to wrap around theresin bed in the KOH generation column. The applied current was variedfrom 1.0 to 70 mA and the operating voltage was monitored. The carrierflow rate was maintained at 2.0 mL/minute.

The dependence of the operating voltage on the applied currentdetermined for the KOH generator operated in three cathodeconfigurations is shown in FIG. 18. At an applied current of 60 mA, theoperating voltage was 45 V when one porous Pt frit was used as thecathode, 40 V when two porous Pt fits were used as the cathodes, and 29V when the cathode was made of a Pt screen formed to wrap around theresin bed. The results indicate that the operating voltage of the KOHgenerator can be decreased significantly by increasing the contact areabetween the ion exchange resin and the electrode, so that KOH atrelatively high concentrations can be generated without being limited byexcessive heating.

EXAMPLE 7 On-Line High Pressure Removal of Hydrogen Gas

An on-line high pressure gas permeable removal device was constructedaccording to the design shown in FIG. 9. A polymeric tubing (0.020-inchOD×0.010-inch ID×1.0 meter length) obtained from Biogeneral Inc. (SanDiego, Calif.) was used as the gas permeable tubing in the device. Thedevice was tested for removing hydrogen gas in the KOH solutiongenerated at applied currents up to 160 mA using the large capacity KOHgenerator described in Example 4. The carrier flow rate for thegenerator was 2.0 mL/minute. In some experiments, the outlet of thedevice was connected to a piece of 0.005-inch ID PEEK tubing thatgenerated a pressure drop of 1400 psi at 2.0 mL/min; the PEEK tubingoutlet was immersed in the deionized water in a small, clear glass vial,and the presence of hydrogen gas in the KOH solution was visuallymonitored (by observing the formation of gas bubbles). In someexperiments, the KOH generator and gas removal device were installed inan ion chromatography system as shown in FIG. 10, the baseline noise ofthe conductivity detector was monitored, and the flow of chromatographysystem effluent was used to shield the outside of the gas permeabletubing to remove the released hydrogen gas and prevent the readsorptionof carbon dioxide from the ambient air, as shown in FIG. 9.

The on-line high pressure gas removal device was highly effective inremoving the hydrogen gas. No hydrogen gas bubbles could be visuallyobserved in the KOH solution generated at applied currents up to 160 mA.FIG. 19 shows the baseline peak-to-peak noises measured at differentcurrents obtained using the device; they are similar to those obtainedwith the conventional ion chromatography system. At the applied currentof 160 mA, hydrogen gas is generated at a rate of about 1.1 mL/min (gasvolume at 14.7 psi). Therefore, the gas removal efficiency of the devicewas quite remarkable, especially considering the fact that the length oftubing used was only 1.0 meter and its internal volume was only 51 μL.

EXAMPLE 8 Use of a Large Capacity KOH Generator in Isocratic andGradient Separation of Common Anions by Ion Chromatography

An ion chromatography system consisting of a large capacity KOHgenerator, an on-line high pressure gas removal device, and commonDionex ion chromatography system components was assembled as shown inFIG. 10. The large capacity KOH generator used was similar to the onedescribed in Example 3. The on-line high pressure gas removal devicedescribed in Example 7 was used. A Dionex AS 11 column (4-mm ID×250-mmlength) was used as the analytical separation column. In isocraticseparation experiments, the large capacity KOH generator was appliedwith a constant current of 40 mA to generate 12.4 mM KOH at 2.0mL/minute. In gradient separation experiments, the current applied tothe large capacity KOH generator was changed from 2.0 to 50 mA in stepsof 0.5 mA per 20 seconds to generate a gradient of KOH from 0.6 to 15.5mM at 2.0 mL/minute.

FIGS. 20 and 21 show, respectively, the representative isocratic andgradient separation of fluoride, chloride, nitrate, sulfate, andphosphate. FIG. 22 shows the reproducible overlay of 16 consecutive KOHgradients generated using the large capacity KOH generator. It is worthyto point out that the chromatographic baseline shift during the KOHgradient was less than 50 nS in the chromatogram shown in FIG. 21. Ifthe same hydroxide gradient is generated using a conventional gradientpump, the baseline shift is usually about 500 to 1500 nS. These resultsdemonstrate that the high purity KOH solutions can be generatedreproducibly using the large capacity KOH generator, and usedeffectively as eluents in ion chromatography. The results also suggestthat the performance of an ion chromatography method can be enhancedbecause the use of high purity hydroxide solution generated on-lineresults in minimal baseline shifts during gradient separation, asillustrated in the next example.

EXAMPLE 9 Use of a Large Capacity KOH Generator in Determination ofTrace Anions in High Purity Water by Ion Chromatography

Dionex Application Note 113 describes a method for determination oftrace anions in high purity waters. In this method, the large volumedirection injection technique is used (sample loop is 750 μL), targetanions are separated on a Dionex microbore AS 11 column (2-mm ID×250-mmlength) using a NaOH gradient. FIG. 23 shows the typical chromatogramobtained when the NaOH gradient (0.5 to 26 mM NaOH) was generated usinga gradient pump and NaOH solutions prepared by conventional means. Thebaseline shift is about 500 nS during the gradient. The baseline shiftoccurs because NaOH solutions are easily contaminated with carbondioxide in the ambient air during the solution preparation and use, evenwith precautions.

To demonstrate the benefits of using high purity KOH eluent generated bythe large capacity KOH generator, an ion chromatography system similarto the one used in Example 8 was assembled. A Dionex microbore AS-11column was used as the analytical separation column. The current appliedto the large capacity KOH generator was changed from 0.4 to 21 mA insteps of 0.4 mA per 17 seconds to generate a gradient of KOH from 0.5 to26 mM at 0.5 mL/minute.

FIG. 24 shows a representative chromatogram obtained for a sample ofdeionized water spiked with 10 anions at levels of 0.9 to 3.0 ppb. Sincethe KOH solution generated with the large capacity KOH generator wasessentially free of carbonate contamination, the observed baseline shiftwas less than 80 nS during the gradient. The significantly smallerbaseline shift during the gradient achieved using the KOH generatorleads to improvements in the method performance. These results suggestthat the performance of an ion chromatography method can be enhanced byusing a large capacity KOH generator.

EXAMPLE 10 Generation of Methanesulfonic Acid (MSA) Using a LargeCapacity MSA Generator Employing a Large Capacity CH₃SO₃ ⁻ Ion SupplyReservoir

A large capacity MSA generator employing a CH₃SO₃ ⁻ ion supply reservoirwas constructed to evaluate this preferred embodiment of the invention.The MSA generation column (7-mm ID×10-mm length) was packed with a20-μm, 8% cross-link strongly basic (quaternary amine functional groups)styrene/divinyl benzene resin in CH₃SO₃ ⁻ form and equipped with a Ptscreen cathode. The CH₃SO₃ ⁻ ion supply reservoir was filled with a 2.0M NH₄CH₃SO₃ solution. The large capacity MSA generator was used togenerate MSA solutions at applied currents ranging from 10 to 100 mA anda carrier flow rate of 1.0 or 2.0 mL/min. The operating voltage for thelarge capacity MSA generator was 9.5 V at 10 mA, 30 V at 50 mA, and 38.5V at 100 mA. The concentrations of MSA generated at 10, 40, and 80 mAwere determined by titration using a 10-mM NaOH standard. FIG. 25 showsthat there was excellent correlation (R²=0.9997) between the appliedcurrent and the determined concentration of MSA generated. In someexperiments, the current applied to the large capacity MSA generator waschanged from 28.5 mA to 70 mA in steps of 1.0 mA per 5 seconds togenerate a gradient of MSA from 17.7 mM to 43.5 mM at 1.0 mL/min. FIG.26 shows the reproducible overlay of 16 consecutive MSA gradientsgenerated using the large capacity MSA generator. These results indicatethat the large capacity MSA generator can be used to generate MSA atdesired concentrations accurately and reproducibly.

EXAMPLE 11 Use of the Large Capacity MSA Generator in the Separation ofCations by Ion Chromatography

An ion chromatography system consisting of a large capacity MSAgenerator, an on-line high pressure gas removal device, and commonDionex ion chromatography system components was assembled. The largecapacity MSA generator described in Example 10 was used. The on-linehigh pressure gas removal device described in Example 7 was used. ADionex CS 12A column (4-mm ID×250-mm length) was used as the analyticalseparation column. The current applied to the large capacity MSAgenerator was changed from 28.5 mA to 70 mA in steps of 1.0 mA per 5seconds to generate a gradient of MSA from 17.7 mM to 43.5 mM at 1.0mL/min. In some experiments, MSA gradients from 17.7 mM to 43.5 mM at1.0 mL/min were generated by using a Dionex GP40 gradient pump withdeionized water and a 100 mM MSA solution prepared from reagent gradeMSA.

FIG. 27 shows a representative gradient separation of 10 cations usingthe MSA gradient generated using the large capacity MSA generator. FIG.28 shows the overlay of two representative chromatograms obtained for ahigh purity water sample spiked with 10 cations at sub to low μg/Llevels, using identical MSA gradients generated with either the largecapacity MSA generator or the GP40 gradient pump. The results show thatthe MSA generator gradient yielded lower detector background and smallerbaseline shift during the gradient than the GP40 pump gradient. Theseimprovements can be attributed to the fact that the MSA solutiongenerated using the large capacity MSA generated is of high purity andfree of contaminants that may be present in the reagent grade MSA.

The results also show that the elution of calcium, strontium, and bariumwere delayed about one minute in the chromatogram obtained using theGP40 pump gradient when compared to the chromatogram obtained using theMSA generator gradient. In the ion chromatography system employing thelarge capacity MSA generator and the on-line high pressure gas removaldevice, the total dead volume of the two device was less than 0.1 mL. Onthe other hand, the GP40 gradient pump used had a total dead volume(consisted of dead volumes in proportioning valves and pump heads) ofabout 1.0 mL. FIG. 29 shows the comparison of MSA gradients generatedusing the large capacity MSA generator and the GP40 gradient pump. Theresults show that the profile of the MSA generator gradient had minimaldelay in the MSA gradient while noticeable gradient delay was observedwhen the GP40 gradient pump was used.

1. A method of generating a base and using the base as an eluent foranion analysis, said method comprising the steps of: (a) providing acation source in a cation source reservoir, (b) pumping an aqueousliquid stream through a first base generation chamber using a pump withan outlet disposed upstream of a first base generation chamber which isseparated from said cation source reservoir by a first barriersubstantially preventing liquid flow while providing a cation transportbridge, (c) applying an electric potential between an anode inelectrical communication with said cation source reservoir and a cathodein electrical communication with said first base generation chamber toelectrolytically generate hydroxide ions in said first base generationchamber and to cause cations in said cation source reservoir toelectromigrate toward said first barrier and to be transported acrosssaid first barrier toward said cathode to combine with said transportedcations to form cation hydroxide, (d) removing the cation hydroxide inan aqueous liquid stream as an effluent from said first base generationchamber, (e) using said pump to pump said cation hydroxide generated instep (d) and a liquid sample containing anions to be detected downstreamfrom said first acid generation chamber through a chromatographicseparator in which anions to be detected are chromatographicallyseparated, forming a chromatography effluent, and (f) flowing saidchromatography effluent, with or without further treatment, past adetector in which the separated ions in said chromatography effluent aredetected.
 2. The method of claim 1 further comprising, prior to step(e), the following step: (g) pumping through a gradient pump one or moregradient eluents into said cation hydroxide eluent stream.
 3. The methodof claim 1 in which said chromatographic separation is performed under apressure of at least 50 psi.
 4. A method of generating an acid and usingthe acid as an eluent for cation analysis, said method comprising thesteps of: (a) providing an anion source in an anion source reservoir,(b) pumping an aqueous liquid stream through a first acid generationchamber using a pump with an outlet disposed upstream of a first acidgeneration chamber which is separated from said anion source reservoirby a first barrier substantially preventing liquid flow while providingan anion transport bridge, (c) applying an electric potential between acathode in electrical communication with said anion source reservoir andan anode in electrical communication with said first acid generationchamber to electrolytically generate hydronium ions in said first acidgeneration chamber and to cause anions in said anion source reservoir toelectromigrate toward said first barrier and to be transported acrosssaid first barrier toward said anode to combine with said transportedanions to form an acid, (d) removing the acid in an aqueous liquidstream as an effluent from said first acid generation chamber, e) usingsaid pump to pump said acid generated in step (d) and a liquid samplecontaining cations to be detected downstream from said first acidgeneration chamber through a chromatographic separator in which cationsto be detected are chromatographically separated, forming achromatography effluent, and (f) flowing said chromatography effluent,with or without further treatment, past a detector in which theseparated cations in said chromatography effluent are detected.
 5. Themethod of claim 4 in which said chromatographic separation is performedunder a pressure of at least 50 psi.